Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS™), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org).
The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations to communicate with wireless communication units within a relatively large geographical coverage area. These base stations are called ‘NodeBs’, in 3GPP.
Typically, mobile/portable wireless communication units, or User Equipment (UEs) as they are often referred to in 3 G, communicate with a Core Network (CN) of the 3G wireless communication system. This communication is via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of Radio Network Subsystems. Each Radio Network Subsystem comprises one or more cells, to which UEs may attach and thereby connect to the network.
Lower power femto cells (or pico-cells) are a recent development within the field of wireless cellular communication systems. These lower power cells have a smaller coverage area than Node Bs. The term femto cells is used hereafter to also encompass pico-cells or similar.
Femto cells are effectively communication coverage areas supported by low power base stations, which may also be referred to as Home NodeBs (HNBs). These femto cells are intended for use within a relatively small area of the cellular network. They may support communications to UEs in a restricted, for example ‘in-building’, environment.
Typical applications for such femto Home NodeBs include residential and commercial, e.g. office, locations, and communication ‘hotspots’. In these uses, a Home NodeB can be connected to a core network of a communication system via, for example, the Internet. A broadband connection or the like may provide this connection.
In this manner, femto cells can be provided in a simple, scalable deployment. They can be used in specific in-building locations where, for example, network congestion at the macro-cell level may be problematic. Significantly, the majority of Home NodeBs are likely to be owned and deployed by members of the public, as opposed to a Network Operator owning a Node B.
One task that needs to be performed within communication networks is ‘Geo-location’. Geo-location is the identification of the real-world geographical location of, say, a UE or the like. Geo-location of UEs can be performed by using network and mobile measurement data for nearby cells. A macro cell typically comprises a substantially unique Primary Scrambling Code (PSC) within its location area. A macro cell may therefore be differentiated from other macro cells, and thereby identified, based on its Location Area Code (LAC) and PSC. Accordingly, by identifying nearby macro cells for a UE, an approximate location of the UE may be determined, based on mobile measurement data for the identified macro cells. However, because of the relatively large coverage area of macro cells, this approach only provides some improvement in the degree of accuracy in identifying the location of the UE.
The use of femto cells for geo-location constitutes a special case. This is because femto cells have very limited range, unlike macro cells, and there can be many femto cells within the physical footprint of a single macro cell. Thus femto cells offer the potential for a higher degree of accuracy in identifying the locations of UEs. However, attempts to use femto cells for geo-location of a UE have tended to experience difficulties, for example:
(i) Femto cells are difficult to distinguish from one another in practice. This difficulty arises since they typically either share a single common Primary Scrambling Code (PSC), or a small number of shared PSCs. This can lead to considerable ambiguity in differentiating one femto cell from another.
(ii) The relatively small number of femto cells has meant that a UE is typically only in contact with a femto cell for much less time than it has a macro cell available. Hence designers have tended to favour geo-location methods that rely on macro cells, rather than femto cells.
Another task that must be performed within communication networks is the ‘hand off’ of a call from one base station to another. The issue of call hand-off is significant, because users of wireless communication devices wish to make seamless calls. This is not always possible in prior art systems. Call hand-off from or to a femto cell is similarly desirable.
Femto cells have often been used to allow a UE to initiate a call, if it is within range of the femto cell when it needs to start a call. Some prior art systems do also allow a call that has been initiated through a femto cell to ‘hand out’, when the UE moves out of range of the femto cell and must communicate through a macro cell. This means that, without the call dropping, the macro cell can take over support of an ongoing call that was previously supported by a femto cell.
However, it is more difficult to ‘hand in’ an ongoing call to the HomeNode B of a femto cell, if the UE moves within range of the femto cell whilst there is an on-going call through the macrocell. This problem of ‘handing-in’ an ongoing call arises partly because of the issue of PSC usage by HomeNodeBs, which makes it difficult for the UE to identify the femto cell that has recently come within range. Some prior art systems are believed to try to hand in an ongoing call to every known femto cell in the footprint of the macro cell. However, this does not represent an optimum use of resources, especially when there are many femto cells per macro cell. Other systems are believed to use GPS location information provided by the UE to attempt to identify exactly which femto cell is in the vicinity, and then hand in to that femto cell. However, this may be inaccurate, for example because there is poor GPS coverage within a building, where the femto cell is located. It also requires that GPS be fitted in the handsets, which also causes additional battery drain.
Thus, there is a need for an improved method and apparatus for identifying a femto cell of a wireless communication network, whereby at least some of the above mentioned problems with known techniques are substantially alleviated.